This invention relates to a device for monitoring physiological signals of a body; and, more particularly, relates to an implantable monitoring device for sensing and/or recording physiologic events with minimally invasive intrusion into the body, but which can be used with various implantable devices.
In using implantable medical devices for recording and interpreting ECG or other physiological data, various other non-physiological signals can be recorded and used to interpret the physiological signals. For example, an automatic trigger signal used to activate data storage, or the noise present when the physiological signal is recorded can be useful in later interpreting the data record. Such non-physiologic signals can also be used to eliminate false indications of medical conditions, and to discover actual problems that would otherwise not be identified. This is particularly true when recording far-field electrogram data, since considerable noise is generally present, and the later interpretation of such signal data will be aided by storing this contemporaneous noise. For instance, in devices that utilize R-waves as a trigger event, it is of particular importance to record contemporaneous noise that may have served as a false trigger. Additionally, in devices wherein storage of signals is patient-activated, it is desirable to store and identify the patient-activation signal as the trigger event.
In the monitoring of long-term ECGs to diagnose intermittent heart irregularities, syncopal events, and other physiological conditions, minimally invasive monitors like the Reveal (TM) electrocardiogram event recorder manufactured by Medtronic, Inc. have proven to be useful. However, particularly when the device employs automatic arrhythmia detection triggers to activate the storage of a segment of the ECG, the presence of noise in the ECG signal channel may trigger activations of recordings inappropriately, causing the device memory to become full of unwanted or redundant portions of the cardiac electrogram which may be of little to no use in diagnosing the patient condition. Moreover, such noise makes interpretation and diagnose of the signal difficult.
Several problems exist with storing information related to the noise and/or trigger event associated with a physiological signal. For example, the amount of available storage must be considered. A separate memory or at least a separate location in memory from the ECG storage area may be required. Additionally, some mechanism is needed to identify which marker was associated with any given segment of ECG data storage.
An additional complexity can be found in the limitation on the nature of the data available to store electrogram data samples, especially when, for one example, the sample rate produces more electrogram features than are stored via a lossy data compression technique in long term monitoring devices, a process relied upon to save memory and achieve sufficient data storage capacity to assist the physician in evaluating a long term ECG.
Monitoring can be done using implantable pulse generators such as pacemakers and other heart stimulating devices or devices with leads in the heart for capturing physiologic parameters, including the ECG. However, the expense and risk from implanting a pacemaker or changing out one without these functions is something both patients and physicians would prefer to avoid. Such devices, in addition to performing therapeutic operations, may monitor and transmit cardiac electrical signals (e.g., intracardiac electrograms) to external diagnostic devices typically with leads fixed in the patient""s heart, to observe electrical activity of a heart. It is common for implanted cardiac stimulation devices to send intracardiac ECG signals to a monitoring device, such as an external programmer, to allow a user to analyze the interaction between the heart and the implanted device. Often the user can designate that the communication from the implantable device to the programmer include a transmission of codes which signal the occurrence of a cardiac event such as the delivery of a stimulation pulse or a spontaneous cardiac depolarization.
U.S. Pat. No. 4,223,678 to Langer et al., incorporated herein by reference in its entirety, discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital (AD) form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry.
U.S. Pat. No. 4,407,288 to Langer et al., also incorporated herein by reference, discloses a programmable, microprocessor based implantable defibrillator that senses and loads ECG data into a memory via a direct memory access operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event afflicting a patient""s heart. Upon such an event, the defibrillator may generate a therapy to terminate the arrhythmia event and store the ECG data sequence of the event, for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
U.S. Pat. No. 4,556,063 to Thompson et al, also incorporated herein by reference, teaches a pulse interval telemetry system capable of transmitting analog data, such as sensed intracardiac electrogram signals, without converting analog data to a digital numeric value. The telemetry system is capable of sequentially transmitting both digital and analog data, individually and serially, in either an analog or a digital format, to a remote receiver. The features and capabilities of such pacemaker/defibrillator devices are now well known, but the problems in long-term monitoring for events and adequate recordation and interpretations of noisy excessively triggered records remain.
An additional implantable arrhythmia monitoring system is described in an article in the December 1992 Vol. 15 edition of PACE (15:588) by Leitch et al. In that article, a feasibility study for implantable arrhythmia monitors describes the use of subcutaneous, bipolar xe2x80x9cpseudo-ECGxe2x80x9d recordings.
U.S. Pat. No. 5,404,887 to Knowlan et al. describes a leadless implantable sensor for cardiac emergency warning that detects heart events through impedance measurement sensed using a coil. A similar system is disclosed in U.S. Pat. No. 5,313,953 to Yomtov et al., incorporated herein by this reference, which describes a large but leadless implant device. With sufficient hardware and connections to the body, numerous other physiologic parameters may be sensed as is disclosed in U.S. Pat. No. 5,464,434 issued to Alt and U.S. Pat. No. 5,464,431 issued to Adams et al., both incorporated herein by reference.
When using the above-described monitoring systems, it may be difficult to determine which type trigger event initiated storage of an ECG segment. The difficulties are exaggerated by the presence of interfering signals, or, in other cases, by the absence of some interfering signals that have been removed by filtering techniques or other anti-noise responses. Moreover, with subcutaneous, or far-field electrodes, ECG signal amplitude may vary greatly with mere change in patient posture, making it difficult to assess whether the recorded signal is a real arrhythmia, or an artifact of poor detection As noted above, this is particularly true when the ECG is reconstructed from a compressed electrogram data.
Therefore, there remains a need to indicate the type of noise that is present in a particular ECG segment, and to do so in an efficient manner within the constraints imposed by the limitations of inexpensive devices with limited communications capacity, battery strength, memory capacity, and having limited time to communicate with external devices, and wherein the storage of the signal is complicated by the use of data compression techniques.
A system and method for storing and communicating information regarding the type of conditions that existed contemporaneously with the recording of an ECG signal is described. As discussed above, during the recording of ECG segments, a variety of information is lost in the normal use of subcutaneous and other ECG monitors. This information includes both trigger events, and/or non-physiologic noise conditions present when the ECG signal is being recorded. According to one aspect of the invention, this information may be captured with the ECG signal and made available to the clinician as screen data or on an electrocardiogram tape recording, for example. This information is stored in a manner that accommodates the nature of the data compression and data communication requirements of the medical device.
According to one aspect of the invention, the type of trigger event that occurred is indicated. In one embodiment, the trigger event may include automatic events such as a Bradycardia, a Tachycardia, or an Asystole event. In another scenario, the trigger event may be a patient-initiated event. This type of stored trigger information may be useful in determining whether the device is functioning properly, as well as providing some redundancy to the vial examination of the reconstructed electrogram segment.
According to another aspect of the system, noise is recorded with the physiological signal, including noise caused by Electronic Article Surveillance (EAS), ElectroMagnetic Interference (EMI) noise, ElectroMyographic (EMG) noise, spurious electrode/tissue movement, pacing spikes, defibrillator spikes, and so forth. A series of filters or filter taps can be used, together with digital signal processing if desired, to determine the nature of these noise signals as they are occurring.
The noise may be categorized in preferred embodiments by using knowledge about the temporal frequency characteristics of the noise. For example, EMG noise is broad band and can be characterized by broadband filters. As another example, pacing and defibrillator spikes are generally high voltage and current and of regularized or expected duration. EMI is generally high frequency and appears in bursts.
According to another aspect of the invention, recorded noise pulses may be used to logically reject future noise present in the high-level arrhythmia detection logic that may be used to automatically trigger an electrogram storage period.
In one embodiment of the invention, the system stores information about other indications of physiologic condition which either contributed to the trigger event, or occurred contemporaneously with the trigger. Whether the trigger is manually-activated or automatic, such information can provide useful information when diagnosing the stored electrogram.
A wide variety of useful adjunct sensors including sensors for edema, pressure, temperature, cardiac output, blood flow, oxygen saturation of the blood, pH, ischemia in the heart, motion or activity, and other sensors may be used with the inventive system. The combining of contemporaneous information from such sensors with triggered electrograms enhances the ability to diagnose the electrogram. It should also be noted that the data can be stored in parallel, if desired, such that two memory buffers can be filled, one with the ECG data and one with the sensor data. This could be particularly advantageous for a pressure wave signal, for instance.
In one embodiment, the information that is recorded in real-time while the ECG signal is being monitored is stored in the ECG memory area as a set of coded markers within the data itself. These markers, which are set to predetermined, off-limit values, replace data points in the compressed and sampled signal.
In the system for retrieval of the ECG segment for display, an interpretive processor in the external device provides an indicator marker in the electrogram where the trigger occurred or where the measurement of the sensor took place. Although when compressed data is stored, some quality in the reconstructed ECG signal may be sacrificed by the recording of the marker indicators, the diagnostic value of this additional marker information compensates for this information loss. In one embodiment, an interpretive processor can complete the ECG display using an intelligence algorithm.
If noise is present in the input signal, the device can respond by eliminating R-wave detection signals or by modifying the patterns of acceptable auto trigger responses to apparent R-waves in the presence of noise, which may affect which segments of the ECG will be recorded. In such cases, no data may be stored if the trigger event does not occur.
According to yet another aspect of the invention, other data may be captured along with noise, including additional physiologic condition sensor data, apparent R-waves which may be used for the arrhythmia triggers, indications of losing contact with the body by the electrodes, detecting pacing pulses, defibrillation pulses, low battery and other internal to the device conditions and so on. Other types of signals that may be recorded include ElectroMyoGraphic (EMG) noise from muscle activity, artifact noise from electrode motion within the body, loss or change in the electrode/body contact, pacemaker pulses, defibrillator pulses, and Electro-Magnetic Interference (EMI), which can be of a wide variety of types from different sources. Any combination of such data may be stored in various preferred embodiments of the present invention.